In internal combustion engines, various combustion faults, referred to as unusual combustion processes, can occur which are not triggered or not exclusively triggered by the ignition spark of the spark plug but rather by spontaneous ignition. This includes, in particular, knocking combustion, premature ignition and autoignitions.
While premature ignition is a sporadically occurring event which disappears again after a certain number of working cycles, autoignitions are self-reinforcing processes of unusual combustion before the usual ignition time. The triggers here are hot surfaces and overheated components such as, for example, electrodes of spark plugs, outlet valves, sharp, over-heated edges in the combustion chamber or excessively hot combustion chamber walls. As a result of spontaneous ignition which takes place too early, the pressure and the temperature in the combustion chamber continue to rise and the triggering component or surface heats up even more. In the next cycle, the autoignition then occurs even earlier and the combustion chamber temperature rises further. This unusual combustion is often associated with pronounced knocking. In the end phase, in which the spontaneous ignition finds its steady-state point, the ignition time is so early that knocking can no longer be perceived. This autoignition cannot be prevented by adjusting the ignition angle.
As a result of the high thermal loading of the combustion chamber, damage or even total destruction of the piston can occur and the electrodes of the spark plug or parts of the outlet valves which are subject to high loading in any case can melt.
In order to avoid the occurrence of combustion faults such as, for example, knocking combustion, document DE 10 2006 015 662 A1 proposes detecting noise in the internal combustion engine by what are referred to as knocking sensors, and the noise for each combustion cycle serves to form cylinder-specific signals. In this context, a monitored cylinder is switched off, i.e. the supply of fuel to this cylinder is interrupted, for example, if knocking owing to pre-ignition occurs, i.e. if the current signal values and the signal values which are specific to this cylinder exceed predefined setpoint values.
EP 0 819 925 A2 discloses a method for detecting early ignition at a spark-ignited internal combustion engine. In this context, a signal of a knocking sensor is acquired at various time intervals. By using the acquired signal of the knocking sensor it is detected whether abnormal vibration is occurring. In addition it is checked whether the frequency of the time intervals of the abnormal vibration exceeds a defined threshold. If this is the case, early ignition is detected.
DE 10 234 252 A1 discloses a method for detecting combustion misfires. In order to improve the detection quality, two different measuring methods are used which operate according to different physical principles. One of the methods detects the rotational speed of the crankshaft, and the other method detects the stream of ions in the combustion chamber.
DE 198 59 310 A1 discloses an engine regulating device which has a pre-ignition-detection device for detecting pre-ignition in at least one combustion chamber. A setting device is provided for setting at least one engine parameter. Furthermore, a control loop is provided between the pre-ignition-detection device and the setting device for suppressing detected pre-ignition by adjusting the engine parameter.
DE 10 2007 024 415 B3 describes a method for detecting autoignition of a spark-ignited internal combustion engine with at least one cylinder which is connected to a crankshaft. According to the method, a rotational speed of the crankshaft is measured during a compression stroke of the cylinder during a first time interval in the working sequence of the internal combustion engine. In addition, a knocking signal during a working stroke of the cylinder is detected during a second time interval in the working sequence of the internal combustion engine. Autoignition of the cylinder is detected if the rotational speed of the crankshaft has slowed down compared to a comparison value, and knocking combustion is detected on the basis of the knocking signal.